The 2012 SDAC showed an estimated 115,400 Australians (0.5%) had autism. This was an 79% increase on the 64,400 people estimated to have the condition in 2009.

In SDAC 2012, Autism spectrum disorders were more commonly found in males than females. Males were 4 times more likely than females to have the condition, with prevalence rates of 0.8% and 0.2% respectively. This is consistent with overseas estimates. For example, the Center for Disease Control and Prevention in the US shows boys are 4.5 times more likely than girls to be diagnosed with an autism spectrum disorder (ENDNOTE 3).

AGE

There was considerable variation in the prevalence of autism across age groups, with a marked drop off in prevalence after peaking in the 5 to 9 years age group (Graph 1). A similar pattern can be seen in the data from the 2009 SDAC. There are several possible reasons for this variation in prevalence across age groups (diagnostic issues, survey scope and methodology) which are discussed below.

CHANGES IN DIAGNOSIS

The criteria used to diagnose autism have been subject to some variation over time. One of the key tools used as the basis for diagnosing autism is the Diagnostics and Statistics Manual of Mental Disorders (DSM). The DSM suggests diagnostic criteria clinicians can use to determine different types ofmental and behavioural disorders. The first two versions of the DSM (DSM (I) and DSM (II)) indicate children with autism should be classified as having ‘childhood schizophrenia’.

Diagnostic criteria explicitly for autism were added to the DSM (III), released in 1980. It was then amended in the DSM III (Revised), released in 1987 and revised again in the DSM IV, released in 1994. The changes from Version III and Version III (Revised) resulted in increased amounts of specific detail that should be considered in the diagnoses. These conditions were revised and reduced for the DSM IV, but at the same time, the number of criteria that had to be met to receive a diagnosis were also reduced. The age specific prevalence rates shown in Graph 1 show a decrease in prevalence for those aged 25 years and over, which corresponds to when the DSM III or earlier diagnostic criteria being used when people in this age group were young.

Recent changes to the diagnostic criteria in the DSM V post-date the collection of data used in this analysis.

It is unlikely however, that people are being diagnosed with other conditions instead of autism as there is no correlating increase in other conditions in the SDAC data that would suggest alternative diagnoses (e.g. other developmental disorders, mental retardation/intellectual disability) in these late teenage years.

THE SURVEY QUESTIONS

Disability identification in the SDAC is achieved by asking about fifteen areas of limitation, such as whether people have anything wrong with their speech or whether they have difficultly learning or understanding things. Social interaction, a common deficit associated with autism spectrum disorders (and a diagnostic criterion), is not amongst these fifteen areas. The majority of people with autism (85%) are identified in the survey because they reported having difficulty learning or understanding. It may be that, as they move out of the education system and into an adult life, the focus on learning on a daily basis decreases and they are therefore not facing the same challenges.

In addition, parents are likely to be the most common reference people answering the disability identification questions on behalf of children living in the same households. As people in their late teens often move out of the family home, a young person in such a situation may become more likely to answer for themselves if surveyed. It is possible that there are differences between what parents report with regard to their children's limitations compared with what the children might report of themselves.

OTHER FACTORS

There are other factors that may also be impacting on the change in prevalence with age. People may be learning coping strategies as they mature and no longer feel they are restricted in any of the areas of limitation considered in the SDAC. Similarly, for some people, intervention therapies effectively remediate the challenges of autism and they are able to function without experiencing limitations in their everyday activities.

It is also possible that people are less likely to identify a child as having autism as they get older because there is some sensitivity around the issue, either for the person or for the other family members.

Whilst none of these points explains the drop in prevalence with age in itself, it is possible they all contribute in some way.